Carbon–Carbon Bond Forming by Ball Milling - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 52

solution 3.6 equiv. were necessary. The authors suggested that the high ef-
ficiency of the solvent-free ball milling method was based on an enhanced
second-order reaction rate resulting from high concentrations of reactants
under solvent-free conditions.
The Diels-Alder reaction of graphite as diene with maleic anhydride and
maleimide as dienophiles was reported by Seo and co-workers. 57 In situ
generated active carbon species (carboradicals, carboanions and -cations),
produced by mechanochemical breaking of graphitic carbon-carbon bonds,
reacted with the dienophile to yield edge-selectively functionalized graphene
nanoplatelets. This reaction was executed in a planetary ball mill at
500 min 1 for 48 h under argon atmosphere.
Wang et al. described the Diels-Alder reaction of fullerene (C 60 ) with
9-hydroxymethylanthracene or 9-methoxymethylanthracene in a mixer ball
mill. After 30 min they achieved yields of 70%. 58 Prolonging the reaction
time to 45 min led only to a slight increase (74%). In contrast, reaction in
solution at room temperature afforded the product in 27% after 24 h.

2.7 Grignard Reaction
Organomagnesium compounds, especially Grignard reagents, have a huge
importance in synthetic chemistry and have a wide range of application. As
nucleophiles they can be used for the formation of carbon-carbon bonds by
reaction with electrophiles like carbonyl groups.
Harrowfield et al. have described the synthesis of Grignard reagents by
milling 1-chloro- or 1-bromonaphthalene with 4 equiv. Mg in a mixer ball
mill under argon atmosphere (Scheme 2.31). 59 The Grignard reagents could
be obtained in 95% yield, which is in same range as solvent-based methods.
In a second reaction step the Grignard reaction took place by adding
benzophenone or frozen acetophenone to the reaction mixture. Yields of
74-80% were realized after milling for further 20 min.
Waddell et al. observed that grinding methylmagnesium bromide in di-
ethyl ether for 1 h with 4-bromobenzophenone yields 1-bromo-4-(1-phenyl-
ethenyl)benzene in 42% and 1-(4-bromophenyl)-1-phenylethanol in 58%
yield (Scheme 2.32). 32 By milling the substrate without solvent for 17 h,
1-bromo-4-(1-phenylethenyl)benzene was formed as the major product.
Therefore,

the diethyl ether was evaporated from methylmagnesium

O

X

R 1

MgX

4 equiv. Mg
MBM
60 s -1 , 1.5 - 2.5 h

R 1

HO

MBM
60 s -1 , 20 min

X = Cl, Br
R 1 = C 6 H 5 , CH 3

Scheme 2.31 Solvent-free Grignard reaction.